Polyethylene has desirable properties that have helped to make it the highest volume polymer manufactured. Ziegler-Natta catalysts are a mainstay for polyolefin manufacture and are used in slurry, solution, and gas-phase polymerizations. Certain applications, such as blow-molding, require a careful balance of rheological properties, and there is a continuing effort to develop polyethylene with improved properties.
Two properties that are important for many blow-molding applications are melt strength and extrudate swell. Melt strength determines how much deformation and sag the parison will experience as it is being formed before mold closing and inflation. When the pressure exerted by the die on the melt is released as the melt leaves the die, the melt swells. As the melt leaves the die, it extends and this parison sag influences parison dimensions and so affects melt swell. While high melt strength and high extrudate swell are desirable, generally extrudate swell decreases with increasing melt strength. When a resin exhibits too little swell, it can become difficult or impossible to properly fill out the extremities of the mold, such as the handle of a blow-molded bottle. Therefore, extrudate swell and melt strength are normally balanced to provide an acceptable combination of physical properties and processability.
Some blow-molding polyethylene grades are prepared from chromium oxide (Phillips) or single-site catalysts and rely on incorporating a high level of long-chain branching into the polyethylene. While some properties are improved, high levels of long-chain branching can hurt other properties. It would be desirable to be able to prepare polyethylene with excellent blow-molding properties with Ziegler-Natta catalyst systems while avoiding tradeoffs of other catalysts. For example, the poorer physical properties caused by long-chain branching and the processability and cost limitations of single-site catalysts are preferably avoided.
One possible solution is to crosslink the polyethylene. For example, U.S. Pat. No. 5,486,575 improves the properties of a polyethylene resin prepared from a chromium catalyst by using an organic peroxide. U.S. Pat. Nos. 4,390,666 and 4,603,173 use peroxides to crosslink a polyethylene blend containing high and low molecular weight components. U.S. Pat. No. 6,706,822 uses peroxides with polyethylene having a broad molecular weight distribution to reduce melt swell. U.S. Pat. No. 5,486,575 uses peroxides with polyethylene prepared with chromium catalysts. While some properties can be improved by crosslinking with peroxides, there are issues with this approach. The radicals produced can interact deleteriously with other additives. It is difficult to predict the effect of crosslinking on rheological properties. Reported results vary significantly from resin to resin, even when the resins are produced using similar catalyst technologies. Peroxides add an extra component to the composition, and they require careful handling and storage, which adds to the cost. It would be desirable to improve properties without using peroxides.
Another approach to improve properties, disclosed in U.S. Pat. Nos. 4,336,352, 5,422,400, and 6,743,863, is to use blends containing three resin components. The blends can be made in a three-step polymerization. U.S. Pat. No. 4,336,352 states that mixtures of high and low molecular weight polyethylene (or blends produced by a multi-step polymerization process) have such a low die swell that blow-molded bottles of consistent quality are difficult to obtain. Their solution includes a three-step polymerization. U.S. Pat. No. 5,422,400 states that earlier approaches with two-component mixtures have important limitations such as the need to use an ultrahigh molecular weight polyethylene having a minimum intrinsic viscosity or the need to prepare such a polymer at temperatures below 30° C. They overcome these limitations by using a three-step polymerization. Unfortunately, a three-component blend or a three-step polymerization is complicated and requires additional equipment.
Polyethylene produced in two-step or even one-step polymerizations can be blow molded. For example, U.S. Pat. No. 6,878,454 teaches films prepared from bimodal polyethylene prepared using a variety of processes from one stage to multistage. The polyethylene has greater than 50% by weight of the high molecular weight component, and it can be extruded at certain low melt temperatures. While melt strength and die swell are not disclosed, they should be low because the approach is similar to that used to make commercially available resins.
Despite continued efforts to improve polyethylene properties for blow molding, there is a need for a process that can produce polyethylene with both high melt strength and high extrudate swell, but which does not require the extra equipment and complications of a three-step polymerization.